Big seamless display screens can display a large amount of data without any visual disruption, e.g. in a panorama format. In the current big screen market, 2 types are generally available: flexible screens or rigid screens. Rigid screens have the burden of transport, packaging, bringing in into a building, and installation. These are generally heavy pieces and quite expensive. An alternative is based on a flexible screen, which has not the above disadvantages, but does not have the best image quality.
Sizes up to 2 m are normally brought in as fully rigid screens. To be able to provide high quality rear projected images of larger size, it is necessary to use to tiled solutions or fully rigid displays that require special logistics. WO 2008/089758 discloses a projection screen that is divided into tiles in order to facilitate logistics and transport.
For projection of high quality images it is important to provide a continuously flat or curved smooth surface without any wrinkles or bulges. Therefore most of the known screens comprise a completely rigid substrate, for example based on PMMA or glass, and the logistics and installation of these big screens are often very difficult: A special entrance into a building is needed, as well as special tooling to install the rigid surfaces, and a number of people involved to perform the logistics and installation. Further, such fully rigid display screens will be limited to the shape they are given at the time of manufacturing. A common way to facilitate logistics of large display screens is to split them up in sub-screens that are put together in a tiled configuration. This, however, brings undesirable physical seams to the screen surface, and it is necessary to align the tiles with some precision adjustment technology in order to minimize those seams. Further, if the screen is used for rear-projection, this adjustment installation must be designed not to interfere with the light path from the projectors.
In particular, at large gatherings of the public such as trade shows and fairs, it is preferred if the screen is rigid as this reduces the chance that members of the public may touch and disturb the screen. With a heavy screen special designs have to be considered since no members can be placed in the active image area for rear-projection. With a more lightweight material the complexity of the frame is reduced but if flexible screens are used draughts in the hall where the screen is located can disturb the screen which is distracting to the viewers. To prevent this a flexible screen may be tensioned but it is difficult to do this perfectly, i.e. without creases or wrinkles and/or to install it in a curved shape.
Rear-projection technology is advantageous for trade shows since there is no risk for shadowing and it provides good contrast ratio. The projectors and supporting optics are put on the backside of the display screen and all space behind the display screen is lost. Thus it is of interest to make these devices extend as little as possible behind the display screen and decrease the system depth which is dead space.
Rear projection solutions for multichannel projection applications can in principal be implemented with optical screen elements and/or diffusive screen elements. Examples of optical screen elements are Fresnel elements, prism elements, lenticular elements, . . . . They are used to combine the output of several projectors into one image and to control the optical performance of the displayed image. A Fresnel element can be used to reduce the hotspot artifact (the hotspot is an enlarged blurred reflection of the projection lens on the screen), a prism element can be used to control the viewing angle, a lenticular element can be used to control the contrast.
The disadvantage is that those optical elements are limited in size and cannot create a single monolithic rear projected image for large screen applications. State-of-the-art solutions therefore make use of a modular concept, where a multiple of physical sub-screens are composed into a large screen.
The difficulties of this technique are that it requires a good matching of the different optical elements, especially with the high resolution projectors currently available. It also requires that the different projected images of the multichannel display do not overlap with each other. Thus no blend zones with a smooth transition between images can be created, and there will be a seam present between the sub-images. In the special case of 3D or stereo images, this seam will negatively influence the immersive experience for the viewer.
Instead of optical elements the multichannel display could be implemented with diffusive screen elements. Such an element could be a rigid or flexible substrate that contains a diffusive layer which will handle the assembly of the sub-images. The large screen could be a single piece of glass, PMMA, . . . covered with a diffusive layer. This solution allows for overlap between the sub-images, and the overlap zones, or so-called blend zones, can be handled with various technologies to a large extent reduce their visibility, for example electronic or optical blending.
The main drawback of solutions consisting of a diffusive display only is however that there is less control of the optical performance. Control of the viewing angle, hotspot and contrast is managed in the diffusive layer which introduces trade-offs in the design of the diffusive layer. For example, improvements on contrast can have negative impact on viewing angle.
Therefore an alternative is to combine optical elements with diffusive screen elements, e.g. using a lens system together with a diffusive screen. For a rear-projection system it would be advantageous to use a Fresnel element before a conventional lens due to its higher collimation power. In this way the distance between the projector and the screen can be decreased, thus the dead space behind the projection system can be reduced. Also there is less hotspot artifact when using Fresnel elements. A display screen involving a Fresnel element has a well-defined focal point and the projector needs to be in this point to provide a collimated beam towards the viewer. The sub-images will however still be displayed next to each other, there will be no zone with overlapping pixels and a seam will be present.
EP1062808 discloses a collapsible presentation screen where the screen material is submitted to a very small radius of curvature.
U.S. Pat. No. 6,842,282 discloses a projection screen without internal support that uses means for tensioning the screen, or alternatively, they construct the screen with smaller shape retaining sub-tiles.
WO0113172 discloses a one piece rollable projection screen that is mounted on a frame that consists of vertical and horizontal members evenly distributed inside the active area of the screen. The part of the screen hanging free is limited to the area enclosed by such frame members.
U.S. Pat. No. 5,897,192 discloses in FIG. 6 a light beam leaving a Fresnel element parallel to the horizontal axis, illustrating that the Fresnel lens is used in its conventional way by producing parallel and non-diverging beams. In this way the light will incident perpendicularly to the screen. There is no overlap between the images since when a Fresnel lens is used for fully collimating to parallel beams, there is no overlap.
U.S. Pat. No. 5,902,030 disclose at least two images that are tiled and spaced less than one pixel apart and projected onto a common Fresnel lens. There is no overlap of images.